1. Field of the Invention
The present invention relates to a liquid crystal display panel made up of structural elements having superior transparency and heat resistance, and exhibiting excellent adhesion to other materials.
2. Description of the Background Art
A liquid crystal display panel is conventionally made up of various structural elements, such as liquid crystals, liquid crystal alignment layers, transparent conductive films, polarizers, optical retardation films, color filters, light guide plates, light diffusing films, and converging films. Along with expanded application of liquid crystal display panels in recent years, including the application to equipment used in vehicles or outdoor, the liquid crystal display panels must be large, withstand severe conditions of use, and produce distinct display images. Because of this, lightness in weight, high productivity, excellent heat resistance and transparency, and superior durability under high temperature and high humidity conditions, without changes in the retardation and deterioration of the film, are demanded of the various structural elements of the liquid crystal display panels. An element which does not satisfy even one of these requirements causes problems, such as indistinct display images, low productivity, inferior compactness, and high cost.
For example, glass has conventionally been used for liquid crystal substrates and transparent substrates for color filters. Because glass is heavy and easily broken, the display panels made from glass are heavy and inconvenient to carry about if they are large, and special care must be taken in handling to avoid breakage of the glass. Furthermore, since it is difficult to roll glass, productivity is poor and glass is not adaptable to mass-production.
In an attempt to overcome these problems, a method of using transparent resins instead of glass for structural elements of liquid crystal display panels has been disclosed. Because films made from these transparent resins are light and flexible, the productivity is improved and the products are light and convenient to use. For this reason, transparent resins are currently widely used for almost all structural elements of liquid crystal display panels. Examples include transparent conductive films prepared by forming a transparent conductive membrane of metal oxide on the surface of films made from acrylic resin such as polymethyl methacrylate (PMMA), polycarbonate resin (PC), polyester resin such as polyethylene terephthalate (PET); polarizing element protective films for polarizers made from triacetyl cellulose (TAC); optical retardation films made from polyvinyl alcohol (PVA), TAC, or PC; light guide plates made from PMMA or the like; color filters, light diffusing films, and converging films made from PET, PC, or the like.
However, all the conventional transparent resins used for these structural elements have some drawbacks. It performance of the liquid crystal display panels are frequently affected by such drawbacks. For example, acrylic resins, PVA, and TAC have a low heat deflection temperature and a comparatively high water absorptivity. Uniformity of film retardation may be impaired by the heat to which the liquid crystal display panels are exposed during the fabrication or by the heat or water during use of the display panels. In the extreme case, the films are deformed. Films made from PET or PC has only poor retardation uniformity. When these are used for liquid crystal display panels, the display images are distorted. PET and PC also have a comparatively high specific gravity and are thus heavy, so that instruments with large panels using a color filter made from these resins are heavy and difficult to carry about. The advantage of high productivity and conveniences, such as compactness and lightness, expected by the use of transparent resins for structural elements of liquid crystal display panels is thus not necessarily realized. It is essential to specially treat each structural element in order to overcome these drawbacks.
Thermoplastic norbornene resins have been proposed as materials to overcome the various aforementioned problems associated with the drawbacks in the abovementioned conventional resins. EP 587890 and EP 591536 disclose a method of using a thermoplastic norbornene resin as optical retardation films, Japanese Patent Application Laid-open (kokai) No. 61026/1993 discloses a method of using a thermoplastic norbornene resin as liquid crystal substrates, and Japanese Patent Application Laid-open (kokai) No. 212828/1993 and No. 51117/1994 disclose a method of using a thermoplastic norbornene resin as polarizing element protective films for polarizers.
These patent applications mention that polycarbonate resins have high water absorptivity and, therefore, their retardation tends to change by absorbing water, and claim that norbornene resins having a water absorption of 0.05% or less can be easily obtained.
However, there are varieties of norbornene resins with different components and these do not always have a water absorption of 0.05% or less. The water absorption of 0.05% or less can be achieved only in norbornene resins made from polyolefins which consist of carbon and hydrogen or those containing some halogen. If a structural element of liquid crystal display panels using a substrate made from such a resin is attached or adhered to other elements, water may invade the interface between them due to insufficient adhesion, whereby the performances of the liquid crystal display panels are unduly impaired.
Although norbornene resins have excellent characteristics, such as transparency, retardation, and low hygroscopicity due to the norbornene structure, they are inferior in resistance to oxygen and heat due to the existence of ternary hydrogen contained in that structure. This is the cause of poor durability of these resins and addition of an antioxidant is essential to improve the durability. The antioxidant is used by blending it with the resin or by applying it to the surface of molded articles. Compatibility of the resin with antioxidants, however, decreases because of the requirement for limiting the norbornene resin components to carbon and hydrogen or to carbon, hydrogen, and halogen for reducing the water absorption. The addition of a large amount of the antioxidant impairs transparency of the resin or causes the antioxidant to bleed on the surface. It has been thus difficult to obtain a resin with sufficient durability.
In spite of the strong demand for high quality liquid crystal display panels made from structural elements, which are light in weight, having high productivity, heat resistance, and transparency, possessing superior durability with no changes in retardation or deterioration of films under high temperature and high humidity conditions, and exhibiting excellent adhesiveness to other materials, no liquid crystal display panels which satisfy these requirements have been known heretofore.
The present invention has been achieved in view of the above problems in the prior art, and has an object of providing a liquid crystal display panel which is made from structural elements, which satisfy all the requirements of lightness, superior heat resistance, transparency, and high productivity, have superior durability under high temperature and high humidity conditions, and exhibit excellent adhesiveness to other materials.